Many cartographic and photogrammetric devices are known for measurement of elevations of the terrain and of heights of objects on the terrain. These devices, commonly known as "stereoscopic plotting instruments", the purpose of which, generally, is to compute positions of points in a three-dimensional space (including elevations), employ mechanical, optical, opto-mechanical and analytical techniques for making these and other photogrammetric measurements, and to record or plot the results thereof.
The most sophisticated stereoscopic plotting instruments are known as "analytical plotters", and are complex, large in size, require specialized training to use and are expensive. Other more simple and less expensive instruments are referred to as "stereometers" (also known as "parallax bars"). Stereometers are used in conjunction with stereoviewers to obtain approximate information about elevations of points in three-dimensional space. The presently available instruments of this type measure stereoscopic parallax differences to obtain data for computation of elevations, and are relatively inaccurate and difficult to use on a continuous basis.
Different types of such instruments and their limitations are described in some detail by Francis H. Moffitt, et al. in Chapters 7, 8 and 12 of the Third Edition text entitled "Photogrammetry", published by the International Textbook Company of Scranton, Pennsylvania in 1980; and in Chapters 10-12 of the Fourth Edition of the text entitled "Manual of Photogrammetry" published by the American Society of Photogrammetry in Falls Church, Va. in 1980.